Temperature dependence of sulfide and sulfate solubility in olivine-saturated basaltic magmas

摘要

The sulfur concentration at pyrrhotite- and anhydrite-saturation in primitive hydrous basaltic melt of the 2001-2002 eruption of Mt. Etna was determined at 200MPa, T=1050-1250°C and at logfO_2 from FMQ to FMQ+2.2 (FMQ is Fayalite-Magnetite-Quartz oxygen buffer). At 1050°C Au sample containers were used. A double-capsule technique, using a single crystal olivine sample container closed with an olivine piston, embedded in a sealed Au_(80)Pd_(20) capsule, was developed to perform experiments in S-bearing hydrous basaltic systems at T>1050°C. Pyrrhotite is found to be a stable phase coexisting with melt at FMQ-FMQ+0.3, whereas anhydrite is stable at FMQ+1.4-FMQ+2.2. The S concentration in the melt increases almost linearly from 0.12±0.01 to 0.39±0.02wt.% S at FeS-saturation and from 0.74±0.01 to 1.08±0.04wt.% S at anhydrite-saturation with T ranging from 1050-1250°C. The relationships between S concentration at pyrrhotite and/or anhydrite saturation, MgO content of the olivine-saturated melt, T, and logfO_2 observed in this study and from previous data are used to develop an empirical model for estimating the magmatic T and fO_2 from the S and MgO concentrations of H_2O-bearing olivine-saturated basaltic melts. The model can also be used to determine maximum S concentrations, if fO_2 and MgO content of the melt are known. The application of the model to compositions of melt inclusions in olivines from Mt. Etna indicates that the most primitive magmas trapped in inclusions might have been stored at logfO_2 slightly higher than FMQ+1 and at T=1100-1150°C, whereas more evolved melts could have been trapped at T≤1100°C. These values are in a good agreement with the estimates obtained by other independent methods reported in the literature.